This invention relates to a method of setting a reference value for judging (whether good or no good) a measured value of a target portion for inspection on a substrate after it has gone through an intermediate process for the production of substrates having components mounted thereto. The invention also relates to a device for carrying out such a method.
The production line for substrates having components mounted thereto (hereinafter sometimes referred to simply as “substrates”) is usually comprised of a plurality of production processes such as a printing process for cream solder by using a solder printer, a component mounting process by using a mounter and a soldering process by using a reflow oven. An inspecting device is provided on the downstream side of at least the soldering process, which is the last of the processes mentioned above, for inspecting the condition of the mounted components and the soldering, as described, for example, in Japanese Patent Publication Koho 3622749.
More recently, substrate inspection systems providing each process with an inspecting device and networking these inspecting devices are coming to be proposed, as disclosed, for example, in Japanese Patent Publication Tokkai 2005-303269. With such a system, the user can use a server or a terminal device within the network to read out and compare the inspection results and images of a same substrate after each of the processes and to thereby analyze the cause of defects found in the final products.
In conventional inspection methods of substrates, it is frequently the practice to use the same reference value of judgment set for another component of the same kind on the same substrate in order to improve the teaching efficiency. There are situations, however, where components of the same kind may have different results in the final process, depending on the conditions under which they are placed, although their conditions may have been similar until an intermediate process. Consider, for example, a situation as shown in FIG. 9 where components 100a and 100b of similar kinds are contained respectively in an area A with a high component density and another area B with a low component density. Since the degree of heat conduction for solder will be different between the areas A and B, the components 100a and 100b may experience different displacements due to the melting solder.
Thus, even if both components 100a and 100b experience about the same positional displacements during a component mounting process, as an example, self-alignment due to the surface tension of the melted solder is likely to come about in the case of the component 100b in area B and hence its displacement is likely to be dissolved but the displacement of the component 100a in area A may not as easily dissolved.
As another example, let us assume that cream solder was applied to both components 100a and 100b in a solder printing process both in an amount equally greater than a specified reference quantity. In such a situation, it may happen that abnormalities do not occur with the component 100a in the area A where solder does not easily melt but that solder flows outside the land and a displacement occurs in the case of the component 100b in the area B.
In other words, even among components of the same kind, their conditions after the final process may become different although their conditions after an intermediate process may be similar, depending on their positions and mounting densities. Thus, if the same reference value is used for components of the same kind in the inspection of an intermediate process, the results of the inspection may disagree with those of the final process.
In view of the above, it is preferable to set different reference values for individual components but the inspection results may fail to agree even if this is done. This is because fluctuations may result in the conditions of substrates after the final process for various reasons such as the temperature changes in the reflow oven even if the conditions up to an intermediate process on components of the same kind mounted at the same position are the same.
Thus, the common practice is to carry out inspections by conceding that it is inevitable that the results of an intermediate process and the final process may disagree to a certain degree and to reset the reference value for judgment only when the frequency of occurrence of disagreement exceeds an intended allowable range.
It is not easy for a person to judge, however, how to set a reference value such that the frequency of occurrence of disagreement in inspection results is reduced to within the user's intended range. For this reason, a conventional method was by trial and error while checking each of the past inspection results but the burden on the user is enormous by such a method.